The present invention relates to hydraulic drive systems of the load sensing control type for civilengineering and construction machines such as hydraulic excavators or the like and, more particularly, to a hydraulic drive system for a civil-engineering and construction machine and to an unloading valve used in the hydraulic drive system, in which the unloading valve is driven in response to a differential pressure between a delivery pressure of a hydraulic pump and a load pressure of an actuator to relieve hydraulic fluid of the hydraulic pump to a tank.
A hydraulic drive system used in a civilengineering and construction machine such as a hydraulic excavator, a hydraulic crane or the like comprises a hydraulic source including a hydraulic pump, a hydraulic actuator driven by hydraulic fluid supplied from the hydraulic source, and a directional control valve for controlling flow of the hydraulic fluid supplied from the hydraulic source to the hydraulic actuator. As the hydraulic drive system, there is a type in which a delivery pressure of the hydraulic pump is so controlled as to be raised by a predetermined value more than a load pressure of the hydraulic actuator. As a representative example of the hydraulic drive system, as disclosed, for example, in U.S. Pat. No. 4,617,854 (corresponding to DE, Al, 3422165), there is a load sensing control (LS control) in which a delivery amount of the hydraulic pump is so controlled as to be raised by a predetermined value more than the load pressure of the hydraulic actuator. In this control system, normally, an unloading valve is connected to a discharge line of the hydraulic pump. The unloading valve has mainly the following two functions: (1) when the directional control valve is in a neutral position and a delivery flow rate of the hydraulic pump is at a minimum flow rate, the unloading valve operates so as to return the pump delivery flow rate to a tank to maintain the delivery pressure of the hydraulic pump at a predetermined value, and (2) when a differential pressure (LS differential pressure) between the delivery pressure of the hydraulic pump and the load pressure of the actuator rises transiently in such a case as when the directional control valve is abruptly returned to the neutral position, the unloading valve operates so as to partially return the pump delivery flow rate to the tank to limit a rise in the LS differential pressure.
Further, in the above-described control system, the minimum delivery flow rate of the hydraulic pump is set to a value larger than a demanded flow rate at the time when the directional control valve is operated by a relatively minute stroke. When the directional control valve is operated by the minute stroke with the intention of minute operation of a working member or element, a part of the pump delivery flow rate is supplied to the actuator, while the remaining delivery flow rate is returned to the tank through the unloading valve.
Furthermore, as another system in which the delivery pressure of the hydraulic pump is so controlled as to be raised by a predetermined value more than the load pressure of the hydraulic actuator, there is a system as disclosed in, for example, U.S. Pat. No. 3,976,097 in which a hydraulic pump of a fixed displacement type is used as the above-described hydraulic pump, and a differential pressure between the pump delivery pressure and the load pressure of the actuator is controlled only by an action of an unloading valve connected to a discharge line. In this control system, when the directional control valve is in the neutral position, a full amount of the pump delivery flow rate (fixed) is returned to the tank through the unloading valve, while, when the directional control valve is operated to the maximum stroke, a full amount of the pump delivery flow rate is supplied to the actuator. When the directional control valve is in an intermediate position between the neutral position and the maximum stroke, a part of the pump delivery flow rate is returned to the tank through the unloading valve in accordance with the stroke position. In the operation at the intermediate position, since the unloading valve normally has a metering characteristic, if a flow rate (a leak amount) returned to the tank increases, the differential pressure (LS differential pressure) between the delivery pressure of the hydraulic pump and the load pressure of the actuator also increases.
However, the conventional load-sensing hydraulic drive systems have the following problems.
In the hydraulic drive systems comprising the above-described unloading valve, a line extending between the unloading valve and the pump discharge line and a line extending between the unloading valve and an actuator load-pressure takeout circuit are different in length from each other and, generally, the latter is longer than the former. That is, the latter line volume is larger than the former line volume. Moreover, the hydraulic fluid as a working fluid has compressibility. For this reason, when the load pressure and the pump delivery pressure vary due to change in the magnitude of the load, change in opening of the directional control valve and the like, a deviation or lag occurs in timing at which these changes are transmitted to the unloading valve as signal pressures, and a delay or lag in transmission, that is, a deviation or stagger in phase occurs between the load pressure and the delivery pressure of the hydraulic pump.
Further, as described above, during operation, the unloading valve relieves a part of the pump delivery flow rate to the tank except that the directional control valve is in the neutral position. Under this operating condition, however, the unloading valve is under a partially open condition, and the LS differential pressure varies depending upon the leak amount of the tank. For this reason, when a phase deviation of the signal pressure as described above occurs when the unloading valve is under such partially open condition, change in position of an unloading-valve spool due to the phase deviation of the signal pressure and change of the LS differential pressure due to the change in position of the spool of the unloading valve interfere with each other. Thus, oscillation occurs in the unloading valve.
When oscillation occurs in the unloading valve, the flow rate supplied to the actuator varies or fluctuates so that operability is reduced. Further, oscillation of a piping system due to oscillation of the unloading valve causes a control lever of the directional control valve to oscillate. Thus, an operator tends to be tired.
In the LS control system in which the pump delivery flow rate is so controlled as to maintain the LS differential pressure at a predetermined value, a part of the pump delivery flow rate is returned to the tank through the unloading valve when the directional control valve operates by the minute stroke, as described above, so that the unloading valve is brought to the partially open condition. Accordingly, in this control system, the unloading valve is liable to oscillate when a minute flow rate is supplied to the actuator. Thus, minute operation of the working element is apt to become difficult.
In view of the above-described circumstances of the prior art, an object of the invention is to provide a hydraulic drive system for a civil-engineering and construction machine and an unloading valve for use in the hydraulic drive system, which are capable of preventing oscillation due to a phase deviation between a load pressure and a delivery pressure of a hydraulic pump which are transmitted to the unloading valve as signal pressures.